Abstract

This paper is concerned with the channeling of a relativistic laser pulse in an underdense plasma and with the subsequent generation of fast electrons in the cavitated ion channel. The laser pulse has a duration of several hundreds of femtoseconds and its power PL exceeds the critical power for laser channeling Pch, with , Pc denoting the critical power for relativistic self-focusing. The laser pulse is focused in a plasma of electron density n0 such that the ratio n0/nc lies in the interval , nc denoting the critical density. The laser-plasma interaction under such conditions is investigated by means of three dimensional Particle-In-Cell(PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is also analyzed by means of a test particles code. The fast electrons are found to be generated by the combination of a surface wave and of the betatron resonance. The maximum electron energy observed in the simulations is scaled as a function of PL/Pc; it reaches 350–600 MeV for PL/Pc = 70–140.

The authors would like to acknowledge helpful discussions with V. Yu. Bychenkov, P. Mora and C. Riconda. The authors would like to acknowledge partial support from the Natural Sciences and Engineering Research Council of Canada. D. Pesme would like to acknowledge a partial support from the French “Centre National de la Recherche Scientique”.

Article outline:I. INTRODUCTIONII. CHANNELING OF THE RELATIVISTIC LASER PULSEIII. ANALYTICAL DESCRIPTION OF THE ELECTROMAGNETIC FIELDS AND COMPARISON WITH 3D PIC SIMULATION RESULTSA. The laser wave fields and their analytical descriptionB. Driven surface waves1. Surface wave dispersion relation2. Wave-number selection mechanism3. Surface wave in the PIC simulation resultsC. The charge-separation fieldIV. TEST PARTICLE SIMULATION RESULTSA. Test particle calculations for the complete set of electromagnetic fieldsB. Electron acceleration caused by the single surface waveC. Electron acceleration due to the Betatron resonanceD. Electron acceleration due to the Betatron resonance and the longitudinal part of the Lorentz force due to the surface wave fieldsE. Electron acceleration by the Betatron-like mechanismF. Energy gain calculationsV. SUMMARY AND CONCLUSIONS